Periphonic or surround sound systems have been developed to enhance the performance of soundtracks from movies, videos and the like, and also a variety of audio recordings.
It has been known for some time to record audio material in a stereo format, but this enables only two channels to be recreated. While this is a considerable improvement of even older monorecordings, it still has a lack of depth, or does not give a true three-dimensional sense to the sound.
Recently, a variety of surround sound systems has been developed. These commonly provide a way of encoding a conventional stereo, two channel sound track or recording with information for four channels. In addition to the standard left and right channels, a centre channel and a rear or surround channel are provided.
The surround or rear channel is provided to give an illusion of space or three dimensions, so as to give a greater fullness and directional quality to the sound.
With left and right channels, it is possible to create a phantom or apparent centre channel by simply providing the necessary signals at equal levels to the left and right channels. However, this has proved to be inadequate or to give a poor effect for many purposes.
In particular, when watching a movie or video, when one or more characters are close to the centre of the screen, the user wants to sense that the sound is indeed coming from the centre of the screen.
The provision of a phantom centre channel has proved to be inadequate and ineffective for this purpose. It often does not provide a strong central image. Further, it is highly dependent on a listener being centrally or equidistantly located with respect to the speakers for the left and right channels. Any listener who is displaced from such a position will experience a strong "pulling" effect to one of the left and right speakers, which gives the disconcerting effect that the sound originates from one side or the other.
This problem has been recognized, and for this reason a centre channel is commonly provided in surround sound systems. The intention is to provide a strong central audio image for any audio portion which apparently originates from the centre or close to the centre of the video image. This is commonly implemented by examining the left and right channel signals, and then assuming that any common signal component represents a central image. This common signal component is then supplied to the central channel and subtracted from the left and right channel signals.
This does indeed overcome many of the problems of a phantom centre channel. A listener experiences an audio image that clearly originates from the centre channel, which will commonly be provided by a speaker located immediately above the screen or video image. The "pulling" problem identified above is also absent, since even an off-centre listener will still experience the full effect of the centre channel.
However, an audio image supplied just to the centre channel will lack all sense of spaciousness or depth, and can have a strong one-dimensional effect. When a listener is listening to a soundtrack having many different audio images, this can produce disconcerting or unrealistic effects. Part of the soundtrack, for example background sound effects and the like will be experienced from the left and right channels, and possibly also the rear, surround channel. This often will be interspersed with speech portions originating just from the centre channel, which will give a wholly different effect. The user will thus hear a soundtrack that rapidly alternates between a surround image coming from all speakers and a mono image coming from just the central speaker.
One common and well-known surround sound system is that developed and made available by Dolby Laboratories Corporation. The two sound tracks are specially encoded, and then decoded using appropriate decoders. An active decoder, marketed under the trade name Pro Logic provides the four channels, left, right, centre and surround. While originally conceived for the film industry, such encoding is now commonly provided on video tapes for home usage, and for decoding in home theatres and the like.
Other workers in this field have identified drawbacks and limitations of known periphonic or surround sound system. An example is found in U.S. Pat. No. 5,172,415 (Fosgate). Fosgate recognizes that a well designed decoder system should provide correct separation, localization and placement of individual predominant sound sources. Fosgate is particularly concerned with the generation and use of control signals that determine signal levels to each channel. Fosgate is more particularly concerned with dealing with extreme dynamic conditions, which can cause control signals to vary rapidly, giving unrealistic effects to a listener.
Fosgate also suggests the use of a so-called "Panorama" control. This is particularly concerned with the balance between front and back particularly in automobile use.
Fosgate identifies a problem with FM reception where, if reception fades, a typical car radio will compensate by gradually blending the left and right channels down to mono, as the signal fades. When such a stereophonic signal is applied to a surround process, the signal is, when at full strength, wrapped around the listener. As it collapses to monophonic, the balance shifts to the front, and hence is far more noticeable to the user.
The Panorama control can alleviate this effect by reducing the initial separation if necessary, all the way down to monophonic. An intermediate position on the Panorama control is intended to provide front to rear balance, by varying the degree to which the stereo signal is wrapped around the listener. At another extreme of the control, the signal again becomes monophonic, but is directed to the rear only.
It will be appreciated that while Fosgate identifies a particular problem and solution in a conventional decoder, this is a particular application to varying FM reception. More notably, Fosgate fails to identify any drawbacks or limitations in the conventional split of signals between the left, centre and right channels.